The Selectric was especially useful for scientific manuscripts, since it could type mathematical symbols. This was possible, since the Selectric didn't use the massive array of type bars of traditional typewriters which limited the maximum number of possible characters. The Selectric used an interchangeable type ball. The surface of these balls was covered with the same impact characters found on the type bars.

A particular character was selected by rotation and tilting of the ball to its position, followed by impact. The mathematical symbols were available by removing the type ball of alphabetic characters and inserting the math ball. It was a tedious process, but it gave employment to many secretaries.

The math ball would need to be inserted quite often for some papers, so type balls were often dropped. Occasionally, a type ball wouldn't work properly after being dropped because one of its index teeth, which were used to mechanically align the ball for perfect spacing, had broken off. I'm fairly sure that I wasn't the first to notice that the breakage was always at the tooth beneath an underscore or dash character (see figure).

In his experiments, Griffith added a controlled flaw, a notch at the surface of the glass fibers. His data led to what's now called Griffith's criterion, that the product of the stress at fracture and the square root of the notch length is nearly a constant, at least for brittle materials; viz.,

σf√a ≈ C

where σf is the stress at fracture, a is the notch length, and C is a constant that depends on the energy required to create two new surfaces at each side of the fracture. Griffith's work was ranked in the top fifty of the greatest materials moments by the The Minerals, Metals & Materials Society (TMS) in 2007.[2]

The Caltech research team fabricated hollow ceramic scaffolds of the same dimension and hierarchical structure of these skeletons, relying on Griffith's prediction that the smaller pieces would have high strength when combined. The fabricated structures (see image) attained a compressive strength of 1.75 GPa without failure, even after many deformation cycles, and their experiments showed that this approach can be used to create lightweight, damage-tolerant, engineering materials.[3]

"Inspired, in part, by hard biological materials and by earlier work... we designed architectures with building blocks that are less than five microns long, meaning that they are not resolvable by the human eye... Constructing these architectures out of materials with nanometer dimensions has enabled us to decouple the materials' strength from their density and to fabricate so-called structural metamaterials which are very stiff yet extremely lightweight."[4]

To fabricate these structures, the research team formed a three-dimensional polymer lattice by two-photon lithography. This polymer lattice was coated with thin layers of titanium nitride (TiN), and the polymer core was removed, leaving the TiN ceramic nanolattice. The hollow struts had wall thickness of 75 nanometers or less.[4]

The research team has fabricated structures as large as a one-millimetercube.[4] They've also substituted gold for the TiN. Says Greer, "Basically, once you've created the scaffold, you can use whatever technique will allow you to deposit a uniform layer of material on top of it."[4]